This invention relates to process monitoring and control by means of a stored program, digital data acquisition and control systems which utilize a master station and a number of remote stations. More particularly, this invention relates to a method and apparatus for ordering the time sequence in which the events at various remote locations occur when there are no synchronized clocks available at the remote stations for accurately time tagging the events in accordance with one time frame.
In the past, the correlation of the time sequence of a number of events at remote stations has required that each of the remote units recording events have a precision clock that is either synchronized with a time standard as by receiving WWVB or is periodically resynchronized to a master time signal. In such systems the correlation accuracy was, of course, based on the drift of the remote clocks since the last resynchronization.
Since the most important aspect of the problem is the accuracy of the overall sequence of events and not the exact time of the day when the events occurred, it is desirable to eliminate the expense and complexity of precision clocks at each remote unit while maintaining high overall accuracy. Thus, it is desirable to use a medium accuracy clock in a way which will minimize the error in time sequential ordering by compensating for clock drift.